Amenorrhea occurs in femal athletes. This unexpected finding raises a number of questions, three of which we will propose to investigate. Aim 1. To determine the endocrinologic mechanisms that cause athletic amenorrhea. The two most likely causes of athletic amenorrhea are weight loss (specifically body fat loss) and emotional stress. We will study these possibilities in a unique group of runners who have intermittent amenorrhea, i.e. during their competitive season when they train many miles each week and they have amenorrhea, but during the off-season when their weekly mileages are reduced, they become regular. We will measure per cent body fat every two months by measuring their body density using Archimedes' principle. The level of anxiety will be evaluated at bimonthly intervals also using a standard psychological test. We propose performing multiple endocrine tests to find clues to the pathophysiology of the amenorrhea. Again we will study the intermittent group of runners during their amenorrheic state and the early follicular phase of their menorrheic state (subjects will be their own controls). We propose the following studies: to determine the 24-hr secretory patterns of LH, FSH, and prolactin; the dynamic testing with GnRH, and the clomiphene stimulation test. Finally we will study the thermoregulatory capabilities of our subjects because the amenorrhea may result from a hypothalamic disorder which may also be reflected in their thermoregulatory capacity. Aim 2. To determine whether athletic amenorrhea affects performance. Because female sex hormones increase ventilation, potentially to inappropriate levels during exercise, we suspect that the absence of these hormones during amenorrhea may alter ventilation in a favorable way. We will determine performnace capabilities and ventilatory variables during the menorrheic and amenorrheic phases of the intermittent runners. Aim 3. To determine whether athletic amenorrhea is a permanent condition and results in irreversible infertility. We will study amenorrheic runners who are college seniors. We anticipate that a large percentage of this group will reduce their training after graduation and become menorrheic. We expect that some of the group will demonstrate fertility by becoming pregnant. These data will support our overall working hypothesis that athletic amenorrheais reversible, adaptive, and is caused by loss by body fat.